I've pretty much covered the minimum amount of thrust needed to sustain a given speed, and the altitude you need to be at allow that minimum value, are you are seriously trying to tell us that because you are *increasing* in speed and altitude things are going to be easier? Significantly?

I haven't said you need to apply this thrust all the time, but almost all that thrust over most of the velocity range. If you try to accelerate as hard as possible early on could you minimise the thrust necessary? Sure - and maybe reduce it by less than one percent because the thrust needed to cruise falls off very slowly, it won't even be that much better for power needed. Less than one percent doesn't really seem to mean much when we're working at such a crude level of approximation. A level which given the numbers involved appears to be entirely appropriate. I won't ask you to explain why ATO takes several days to reach orbit when small differences in thrust/power would change that value entirely.

What you haven't shown and won't be able to show is a way to reach orbit with meaningfully less thrust(or power) than that suggested by just the bare L/D. Diving and climbing are not convincing ways to increase energy at a constant thrust in a medium where the influence of drag is pervasive. Nor is circling which explicitly wastes lift using it for centripetal acceleration.

LukeSkywalker wrote:

You keep returning to the idea that the balloon-ship will need to go super or hypersonic, against non-negligible densities of air.

It is also explicit that you must be much hypersonic by the time you have shifted much weight to centripetal acceleration. This necessarily involves supporting most of the vehicle weight aerodynamically at very high altitudes and with air-densities we would ordinarily describe as negligible but because of the extraordinarily low density of the vehicle turn to not be at high speed.

LukeSkywalker wrote:

Atmospheric ion propulsion

Lourens vehicle would need 40MW at 5000m/s with an L/D of 10(!!) [force=power/speed] if it uses external reaction mass irrespective of how it works and even then only if it is 100% efficient. If you had moved over to a lower ISP rocket thrust before this point you can do it with less power (ISP in hydrogen resistojet range) but you can't have been using that engine for much of the time as you would rapidly run out of fuel. You can't obtain propellant externally and get the same energetics. The improvement in power requirement is only possible due to shedding kinetic energy of the already high speed propellant and the slight improvement given by reducing vehicle mass. You must already have got rid of the hydrogen used in buoyancy by this altitude, hydrogen propellant here must be extra and you need a large amount of it.

The altitude I gave for 5000m/s just represents the altitude at which you need minimal thrust to cruise at, if you have a lot more thrust available you can reach that speed at a slightly different altitude, but you can accelerate *much* harder if you try to follow the minimum drag path even if you are using a higher thrust over part of it. It isn't a significant speed if you are using rocket power to generate thrust, but it is very significant if you are driving external air and are power limited.

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move only the air directly around the ship, so that you effectively use your energy to overcome friction

Such extremes of technology are equally unlikely to materialize in seven years.

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And who says props are out of the question?

I have already said that you can gain some speed and altitude using props or ducted engines, but you cannot use them beyond a certain (low) speed as they quickly thereafter become drag-only devices.

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we'd need to do some real, solid aerodynamic analysis rather than conjectures.

I am extremely confident that the more aerodynamic scrutiny you put ATO under the worse it will look and not by any small amount.

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As for JP possibly withholding some crucial information, that seems unlikely.

Like where its personal space elevator is?
Nothing short of levitation or L/D around a hundred logically fits with a very lengthy and time symmetrical ascent and de-orbit.

I've pretty much covered the minimum amount of thrust needed to sustain a given speed, and the altitude you need to be at allow that minimum value, are you are seriously trying to tell us that because you are *increasing* in speed and altitude things are going to be easier? Significantly?

Yes, you covered it, but I'm claiming the equation you're using to cover it isn't applicable. I could be wrong about that, but I reviewed my previous post where I attempted to explain it in more detail and failed to find the mathematical error that belies my foolishness.

As for the ATO's ascent becoming easier with altitude, nothing could be further from the truth. Analysis of the radially outward forces on the ATO (Fup) during its ascent, where:

Fup = L + B + Iy + Dy

reveals that the maximum upward force on the craft comes early in the flight. Then, it's all downhill from there. For a long stretch of the ascent, retaining upward force gets harder the longer it flies. Bouyancy falls, drag increases, and even aerodynamic lift tapers off. The point of minimum upward force is reached well before orbital velocity. If the ATO can't ever burn off enough fuel for its weight to fit under that minimum force, it can never ascend to orbit.

nihiladrem wrote:

Diving and climbing are not convincing ways to increase energy at a constant thrust in a medium where the influence of drag is pervasive.

Well, Solomon Andrews's Aereon, the world's first "self propelled" airship, flew that way. Unfortunately, to make headway in this fashion the ATO would need to dive at a rate faster than the prevailing wind aloft. The Ascender sketches released by JP Aerospace don't appear to include any "Sit Down, Shut Up, and HOLD ON!" bumper stickers, so I think that no diving and climbing is a safe assumption.

In the name of preserving the sanctity of tea time on the Orbital Ascender, I'll go with you on this one.

nihiladrem wrote:

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we'd need to do some real, solid aerodynamic analysis rather than conjectures.

I am extremely confident that the more aerodynamic scrutiny you put ATO under the worse it will look and not by any small amount.

Hmm... All right.

_________________“The next generation of engineers, astronauts and scientist are not going to appear out of thin air. They need to be inspired and educated, and the best way to do that is to get them involved.” - John M. Powell

Regarding the question if ascent becomes easier at higher altitudes: At higher altitudes the density of air is less - there are less molecules per cubemeter. If the engine is switched on the ATO is moving - it suffers friction by the molecules of the air but since the density of air, the number of molecules per cubemeter decreases with altitude friction is decreased with altitude. This should make the ascent easier under propulsion. What you say seems to me as if it is valid for the ascent from 42 km altitude to 60 km altitude which seems to occur without propulsion according to the .pdf at JP Aerospace's homepage.

Regarding the question if ascent becomes easier at higher altitudes: At higher altitudes the density of air is less - there are less molecules per cubemeter. If the engine is switched on the ATO is moving - it suffers friction by the molecules of the air but since the density of air, the number of molecules per cubemeter decreases with altitude friction is decreased with altitude. This should make the ascent easier under propulsion.

Ekkehard this is only half the story, the density of air also reduces the bouyancy of the craft which has to be compensated by extra thrust, what Nihiladrem is saying that at some point before it reaches orbit the vehicle will need more thrust than it can reasonably be expected to produce.

_________________A journey of a thousand miles begins with a single step.

As for the ATO's ascent becoming easier with altitude, nothing could be further from the truth...

This shows realism, but I'm not sure what part of my post you're referring to and I think you may have got the wrong end of the stick. What I've been saying has been more or less this.

The minimum drag altitude for a given speed is a simple function of speed and increases with speed. If you try to work optimally, the general rule is that the faster you go, the higher you need to go. If you have minimum thrust you cannot keep this speed at any other altitude. When you start the ascent, everything gets worse as speed (and hence lowest drag altitude) increases. First you lose buoyancy, then L/D begins to suffer. Everything gets worse at altitude increases.

However once you are going very, very fast the thrust needed and later also the power needed if you use external reaction mass beings to decrease. Right at the point when you are reaching extreme altitudes. Then everything gets better as altitude and speed increases.

It's been brought up several times that there might be some way to get from the good situation at very low speed to the good situation at very, very high speed without a bad situation in the middle. I've tried to show that the region where things are good is very small and in general the whole ascent needs too much thrust and too much power regardless of how you attempt to sneak past needing a particular value of thrust or power.

In line with using more aerodynamic scrutiny you can show using basic trig that at very low L/D the engine thrust vector should point somewhat above the horizontal, but the benefit at higher L/D is rather small. At L/D of 5 you can get a 2% reduction in thrust needed over what bare L/D suggests using horizontal thrust. The improvement is much less with a L/D of ten. Higher L/D is still always better, so the general picture doesn't change.

Wings optimised for lift at extremely high speeds have a section which resembles a sharp, low-angle wedge (L/D being limited by the wedge-angle amongst other things).

Interesting, was this intentional or just due to an absence of adequate control surfaces at full power? I have observed it happening in a crude simulation but I wasn't trying to actively stop it.

BTW. There is an extra term needed in the force-analysis in non-cruise conditions, the climbing rate vy results in a fictitious retarding force fx=-m*vx*vy/r. Without this an object without the influence of gravity and aerodynamic force will not conserve kinetic energy, moving in a straight line at constant speed. You can think of this as the centrifugal force observed in this coordinate system not being a free lunch. [I'm not sure if your Ix term already includes this or not..]

BTW. There is an extra term needed in the force-analysis in non-cruise conditions, the climbing rate vy results in a fictitious retarding force fx=-m*vx*vy/r. (I'm not sure if your Ix term already includes this or not..)

Yes, my present calculations include it. I wish it were fictitious. This web site provides a derivation of the vector equation for acceleration in spherical coordinates. Subtract that vector equation from the equation for straight line acceleration, and what's left over is the various centrifugal and coriolis accelerations. Their energy is provided by gravity, drag, etc., and the vx*vy component you mentioned is represented by some of the positive terms in aTHETA and aPHI.

nihiladrem wrote:

The minimum drag altitude for a given speed is a simple function of speed and increases with speed. ... If you have minimum thrust you cannot keep this speed at any other altitude. When you start the ascent, everything gets worse as speed (and hence lowest drag altitude) increases.

During the middle range of the ascent - probably the bulk of it, time-wise - yes. On top of that, courses that allow the use of solar arrays for power impose further constraints on velocity, since they have the ship racing the sun. The middle range of the ascent is definitely the hardest part that I can see. Most of the vehicle's time will be spent there, most of its fuel will be burned there. I believe it should be able to slowly gain altitude the whole while... but fast enough? Even if it works, most of the ATO's flight is going to be a delicate balancing act.

What happens outside that middle range is still of interest, though. From what I can see, the flight conditions under which available upward force greatly exceeds the weight of the vehicle occur at the beginning and end of the flight. At ignition, the vehicle is in equilibrium - a dropped screw will set it in motion. And orbital insertion should occur at significant acceleration, too, with plenty of aerodynamic lift left over. JP may not be joking about that sudden outbound turn to Mars.

Interesting, was this intentional or just due to an absence of adequate control surfaces at full power? I have observed it happening in a crude simulation but I wasn't trying to actively stop it.

Aereon was a real airship. It flew back in the 1800's, before they had powerplants strong enough to fight the wind. It was sail powered. Useless as an ATO, but I'd love to have one.

I've seen the same porpoising phenomenon in my own crude simulations. A little extra numerical integration fixes that right up.

_________________“The next generation of engineers, astronauts and scientist are not going to appear out of thin air. They need to be inspired and educated, and the best way to do that is to get them involved.” - John M. Powell

Looks like we pretty much discussed the heck out of this topic. We need to hack the NASA databanks and dredge up upper atmospheric data from sounding rockets and stuff and see if any special conditions (such as daily changes in the density of the upper atmosphere) could prove advantageous. That may be something to consider. Or how about digging up some good wind-tunnel data for v-shaped blimps?

Gee, I just don't know enough to really decide one way or the other, and I'm sick of waiting. HURRY UP, JP!!! lol, just kidding. Take your time, I know that's important. Say, I wonder if JP needs someone to scrub the decks on the first longterm DSS mission? Who says you can't work your way to the stars, as a janitor on a spaceship?

Speak for yourself! We haven't even gotten around to the mass ratio yet.

Good point about upper level winds, though. There's no need to hack anything - lots of good data out there in the open. For example, try the Global Scale Wave Model home page. And don't forget to check out the sweet spots - perpetual mesospheric and thermospheric updrafts. If the ATO can swing its trajectory through those a few times, it could be ratcheted into the sky with each pass. They're not enough to account for the ascent all by themselves, but every little bit of extra energy helps.

LukeSkywalker wrote:

Say, I wonder if JP needs someone to scrub the decks on the first longterm DSS mission? Who says you can't work your way to the stars, as a janitor on a spaceship?

Darn right! Why, if I were JP, I wouldn't stand for a blue ice problem at 140000 feet. The DSS will need clean toilets just like everything else. I'll sign up with you.

_________________“The next generation of engineers, astronauts and scientist are not going to appear out of thin air. They need to be inspired and educated, and the best way to do that is to get them involved.” - John M. Powell

I haven't done my own calculations on the trajectory yet (and I wonder what an inertial force is, and whether F = m * v really holds and who is going to get the Nobel Prize for that discovery), but I was reading through the aforementioned aerodynamics tutorial, and in the section on high-speed flight it mentioned that in hypersonic flight, drag greatly increases because the air around the ship gets so hot it gets ionised.

Someone mentioned an atmospheric ion drive. Well, you've got the ions. So, how about sticking a metal grid into the air flow at the bottom of the craft, and connecting those solar cells to it. I'm not sure whether this kind of ionisation results in the normal O(2-) and N(3-) anions or whether the electrons just get stripped off and we get cations instead (seems more likely), but the principle remains the same, you just connect the wires the other way around.

Air flows towards the craft, hits it and flows along the bottom, heats up, and ionises. Assuming the ions are positive, they are accelerated towards the negative grid at the back of the craft. The electrons move the other way, but they are so much lighter that the impulse effect of that is negligible.As long as the solar cells keep up the charge difference between the grids, the ions get accelerated and the vehicle gains impulse. In fact, you don't even need a current, just a voltage, as long as the passing ions don't take electrons away from the negative grid.

The secret is that the situation is asymmetric: the air only gets ionised when it gets between the grids, so there is no deceleration of the air before it passes through the foremost grid, but there is an acceleration when it gets between the grids. Of course, it decelerates again after the last grid, but that still gets it twice as much acceleration (push from the first grid, pull from the second grid) as deceleration (pull from the second grid after it's passed it).

In fact, the construction can be even more simple. Create a big flat plate out of solar cells that forms the bottom of the craft. Stick the grids (you can have a whole row of them, closer together for a stronger electric field at a cost of more drag) directly to the bottom of those cells. Add a big (transparent!) PE bag on top (possible double-walled and vacuum filled) and you have an ATO.

You do need to get it up to Mach 5 first, though. Or maybe we can add some kind of ioniser between the grids. Does this make sense to you?

lololol! I retract that statement. I have added about as much as I can to the discussion. Sounds like it's not over, though.

Hmm. Yes, my skilz at searching the internet are not complete. Obi-Wan has NOT trained me well. I hadn't thought of using thermals, in fact I thought there wasn't supposed to be any air movement up there! Fun stuff.

Lourens: Working with ion propulsion is going to open some major cans of worms. I think you are right about the air getting ionized, but the amount of energy that is soaked up by that process is probably very high, possibly more than you will get by accelerating the ions in your grid-produced feild. But ATO may not even need to be going that fast to begin ion production: I think at high altitudes there is already a significant fraction of ions due to solar and space radiation.

One critical problem with Ion thrust is the way the thrust is produced. At sea-level pressures, what happens is that only a small fraction of the air is ionized, and it gets pulled along by the field gradient, which draws energy from the grids as force*distance. The ions drag the rest of the air mass along with them by bumping into the other molecules. Somehow, and I am really not sure why, this mechanism is extremely inefficient. I don't know if efficiency would be increased or made worse by operating in more rarified and ionized conditions.

I don't know if traveling faster would help or harm the efficiency. One on hand, I think that the same amount of thrust can be sustained from a constant power supply using ion thrust no matter what the ship speed, meaning an increase in thrust efficiency. On the other hand, the dynamics of the ions may get worse, especially at supersonic speeds.

Lourens said:

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In fact, you don't even need a current, just a voltage, as long as the passing ions don't take electrons away from the negative grid.

The secret is that the situation is asymmetric: the air only gets ionised when it gets between the grids, so there is no deceleration of the air before it passes through the foremost grid, but there is an acceleration when it gets between the grids. Of course, it decelerates again after the last grid, but that still gets it twice as much acceleration (push from the first grid, pull from the second grid) as deceleration (pull from the second grid after it's passed it).

That's not quite right: the ions must be neutralized by gain the electrons lost at the fron grid, which requires a current. Besides, the way your suggestion would work, it would make a perpetual motion machine.

The upper winds and solar cells mentioned caused me another thought which isn't related to those both but might serve as a general idea.

The ATO seems to be considered here as passively reacting to atmospherical and energetical conditions etc.

What if it would handle them actively?

The ATO is going to be very large - the wings will be longer than a mile each. If the total area of them also is significantly larger than the usual area of wings then this might have in impact on the local atmospherical conditions around the ATO.

The air below the wings might be cooled down and descending while the upper side of the wings might heat the air above the wings and make it go up. This could reduce friction.

It copuld be reverse instead - if the sun shines onto the lower side of the wings the air might be heated there and push the ATO to higher altitude while the air above the wings might be colled down and descending to lower altitudes. This causes decreased density at higher altitudes and this way decereased friction. The angle the ATO is going by may cause the cooled air flow down the wings while the heated air would flow upwards along the lower side of the wings.

Air that has passed the ATO going down would be hetaed there again and it might be possible perhaps to make it go to the regions just below the wings and contribute to the updrift there.

That's an interesting thought Ekkehard. If the airship is a flat plate with a transparent bubble on top, then the sun could heat the bottom plate through the bubble, and that heat could be radiated downwards. That heats up the air underneath the craft, creating an updraft.

Altitude is only a small part of the problem though. We still need to get up to orbital speed.

Luke: The air will get ionised anyway, and yes, it soaks up energy, which is why drag increases. Or at least that's how I understand it. However, if you're going to fly at hypersonic speeds, there is no way around that anyway. So why not use it?

The perpetuum mobile problem had crossed my mind, and I wasn't quite sure whether this constituted one. I figured I'd post it, and one of you would point out the flaw . My idea was that when the ions are created, they immediately have potential energy, because they are in the field. Normally, it takes as much energy to get the charged particle to where it is as you can get out of it by letting it fly out again, but if you can turn off the effect of the field on the particle while putting it in place, and then turn it on again...

Consider an analogy with gravity. Imagine you have a special brick that you can make weightless. You make it weightless, carry it up a mountain, then make it weigh something again and let it fall down...that really is a perpetuum mobile, so it's not going to work. Erm.

Ah, crap. I've got it, that analogy is flawed. The electrons get pushed forward, with the same force as the ions. They may have a lower mass, but that just means they get accelerated to a larger velocity. The amount of impulse on the electron is the same as on the ion, so the total amount of impulse added to the electron is the same as that added to the ion, only in the opposite direction. Total effect on the vehicle: zero. Which makes sense.

Conclusion, the ions need to be caught by the grid at the front, transported through the solar cells to the grid at the back, and added to the stream again there. That means the cells need to provide power, and we no longer violate the laws of thermodynamics. Phew .

Now the question is whether we can catch those electrons in our grid while they are zipping by...

By the way, can we dub this the outboard plasma drive? Do you think we need to mix in some oil with those ions?

That's an interesting idea, Ekkehard. I had not considered that the orbital ascender (or DSS, which is even bigger) might make its own weather. However, I don't forsee that effect being significant to the orbital ascender once it exceeds the speed of sound. The airflow gets too hot compared to solar heating, and it simply moves out of the way too quickly.

Lourens, I see no need for a plasma skin or atmospheric ion drive. In fact, there are two very good arguments against it.

First, there's already the potential for plasma temperatures in excess of 3000 degrees celsius on the orbital ascender's skin when it's hypersonic. I can't see how adding the extra energy to make that 6000 degrees is going to improve the situation. Also, my calculations indicate that the ATO will not fly if its mass isn't time variable. The weight has to be reduced to less than the minimum upward force at some point, and that minimum force is less than the starting weight. Thus, the orbital ascender must shed mass to make that happen. And if you've got to throw it off anyway, why not burn it for fuel rather than drop it as ballast?

BTW, Nihiladrem, it occurs to me that while the vehicle's performance isn't accurately described by L/D, it is still accurately described by D/W (equivalent to F/W in the conventional airplane model). Thus, when its D/W ratio is unfavorable for further ascent at any given point, the ship can just hang there until it burns off enough fuel to make it favorable.

Give the ATO a large enough mass ratio and point it in the right direction, and eventually it will go up, even at lower L/D values. The minimum possible mass ratio would be

R > WSTART / Fmin

There are parameter sets with a ridiculously high R, of course, but we're not interested in those. I think R=3 or 4 is a realistic proposition.

_________________“The next generation of engineers, astronauts and scientist are not going to appear out of thin air. They need to be inspired and educated, and the best way to do that is to get them involved.” - John M. Powell

What has been said about the creating of its own weather by the ATO seems to be right and correct.

It has been said that the accumulation of several very small positive effects on velocity and lift might get the ATO the required velocity to achieve an orbit - that own weather of the ATO might be such a small positive effect: the own weather causes speed upwards while the electric drive causes velocity to directions angled to "upwards".

Both the vectors of theses two speeds/velocities added on result in increased velocity.

The effects of the own weather will be there during several days...

It seems that in principle the DSS could enable such own-weather-effects at the night-side of Earth too - if it would be equipped by mirrors to refelct sunlight to the ATO when it is on the night-side.

There is a point that mustn't be kept out of consideration. When the ATO leaves the DSS it will already have some speed - the DSS has speed it gets by the current it is in (mentioned in the www.jpaerospace.com -thread).

So the ATO docked to the DSS will have that speed too - and it will keep it by far after undocking.

The electrical drive provides add-ons to this initial speed - as well as the possible vertical speed-component contributed by the effects of the ATO's self-created weather.